Several researchers are investigating the drink makers' claims about their products.

The New York Times reports that the human studies all have a similar conclusion: increases in alertness occur because the drinks have a lot of caffeine. One researcher calls the drinks "caffeine delivery systems". And, they're costly: about $3.00 for a 16-ounce can that has as less caffeine than a cup of Starbucks coffee costing less than $2.00. Or, about the same amount of caffeine as a NoDoz tablet costing $.30.

Sales of energy drinks in the U.S. last year totaled more than $10 billion.